Proper operation of high-performance synthetic aperture radar/ground moving target indication (SAR/GMTI) radar systems requires precise measurements of platform motion (e.g., motion of an aircraft). This is commonly performed with a global positioning system (GPS)-aided Inertial Navigation System (INS). Integral to an INS is an Inertial Measurement Unit (IMU), whereby the IMU is typically composed of three orthogonal accelerometers and three orthogonal rate gyroscopes. The task of the GPS is to provide absolute references for correcting errors which can occur at the IMU owing to noise, drift, etc.
The combining of the GPS and IMU data to estimate errors and corrections, and to achieve a blended motion measurement solution, is often performed utilizing a linear quadratic estimation (LQE) such as a Kalman Filter (KF), an Extended Kalman Filter (EKF), etc. Such an algorithm and its implementation are frequently termed the “navigator”. The correction of IMU motion information is termed “alignment” of the navigator.
The GPS can provide data to correct errors in accelerometer data. Roll and pitch gyroscopic errors can be corrected with the additional knowledge of gravity, which always accelerates downward. However, gyroscopic yaw errors are not observable and hence are not correctable when a radar system is in straight, level, and constant velocity flight. Yaw error is also known as “heading” error, with heading error being an orientation error, as opposed to a translational error due to direction of platform flight.
Conventionally the complete alignment of a GPS-aided IMU navigator during flight requires substantial horizontal accelerations to make the heading error observable. Such accelerations are typically sustained for a number of seconds (for example, 10-20 secs) with a magnitude in the order of about 0.5 G in the horizontal direction. Aircraft typically perform maneuvers such as S-turns or circles to accomplish the required acceleration. For a ‘tactical grade’ IMU, such an S-turn might be required, for example, every 15 minutes or so, thereby potentially disrupting the mission of the aircraft as well as negatively affecting passenger comfort. Linear accelerations can also be utilized to accomplish IMU alignment, but their application on board an aircraft is limited.
Other techniques do exist to measure heading error and facilitate alignment of the radar navigator. Some techniques are better suited to correct for small errors, other techniques are better suited to correct for large errors, while some techniques can do both. One class of techniques involves the employment of additional instruments for detecting platform (e.g. aircraft) attitude, and ultimately attitude of a radar system located on the platform. An example of such an instrument is a digital flux-gate compass. However, such compass systems can be very sensitive to extraneous ferrous metal and difficult to mount in a manner to provide a desired accuracy.
Another example is a GPS-based Attitude Determination Unit (ADU) which employs multiple widely spaced GPS antennas with differential measurements to determine platform attitude. However, an ADU system requires additional hardware beyond that utilized in a typical radar GPS-aided IMU installation. This in turn represents additional cost and complexity, as well as size, weight, and power demands on the platform. A particular platform may, for example, not allow the mounting of the widely spaced additional antennas required for the ADU.
The employment of a higher grade IMU, for example a ‘navigation grade’ IMU instead of a ‘tactical grade’ IMU, may facilitate holding an acceptable alignment for many hours. However, this comes at a cost of increased price (often by a factor of 5 to 10), as well as additional size, weight, and power. In addition, an initial alignment is still required by conventional techniques, such as an S-turn. However, some aircraft are not capable of S-turns that can generate sufficient horizontal accelerations. Examples of these are many dirigibles and blimps.
Measurement and correction of a navigator heading error while in flight by analyzing SAR images is also available. While such an approach can work for small heading errors, it is not able to perform an initial alignment for a large initial error. Essentially, the technique performs well for keeping an IMU aligned once an initial alignment has been achieved. However, as with the navigation grade IMU, an initial alignment is still required by a conventional technique, such as an S-turn.
A well-known technique for providing an initial alignment to an IMU is termed a “ground alignment” whereby the aircraft is stationary on the ground for some period of time (perhaps 15 minutes) at a known location allowing the IMU to sense the rotation of the earth, and orient itself accordingly. However, this technique has limited accuracy and precision especially for tactical grade IMUs, and furthermore precludes performing an initial alignment while in flight, such as might be required with a system restart. Landing an aircraft for each system restart is generally not feasible.
While in flight, a navigator can be aligned to the body of the aircraft, but there is no guarantee that the body is pointed in the exact direction of flight. The angular difference between the front of the aircraft body and the direction of flight is termed the “crab” angle of the aircraft, and is generally unknown due to unknown winds aloft (both in terms of speed and direction). Crab angles can be particularly large for large slow-moving aircraft such as many dirigibles and blimps.
Consequently, it becomes desirable to perform an initial alignment of the GPS-aided IMU navigator while the radar is in flight, where the initial alignment doesn't employ any additional instrumentation beyond the normal GPS-aided IMU navigator and perhaps the radar system itself, and can be accomplished during straight, level, and constant velocity flight.
One technique that utilizes a radar system to determine direction of flight with respect to an aircraft body is embodied in a class of radar systems called Doppler navigation radars. Such radar systems utilize multiple beams directed in different bearing directions with respect to the aircraft. The beams are generally transmitted with fixed angular differences. The Doppler information of these beams is compared to determine the direction of translation of the radar over the ground with respect to the aircraft body. Doppler navigation radars are custom systems that represent additional equipment with additional cost and complexity, as well as size, weight, and power demands on the platform.
Another navigation technique using radar is known as Terrain Contour Matching (TERCOM). This technique uses a radar altimeter to generate a height profile of the ground below an aircraft, and then attempts to match it to a pre-recorded contour map of the terrain. While this enhances the position and velocity information available to the navigator, it does not address angular yaw or heading errors.
More generally, fiducial targets, such as known landmarks, can be employed to determine heading errors, but this requires prior knowledge of those landmarks, or limits the region over which an alignment can occur. A variant of this is to match specific radar echoes to data reported by other means, such as Automatic Identification System (AIS) reports. This, however, again requires additional instrumentation, and of course the presence of suitably equipped craft.